This invention relates to methods and apparatus for providing images of extremely small spacing gap variations within an area between mutually facing surfaces and, more particularly, it concerns a method and apparatus for displaying an image pattern representative of spacing gap variations between a glass surface and another stationary or moving surface on a real time basis, and wherein the image pattern can be calibrated to measure spacing gaps in the range of from about 18 microinches down to less than 1 microinch in increments of less than 0.5 microinches.
The magnetic recording media industry exemplifies a field in which there is an acute need for accurate measurement and/or observation of extremely small spacing gaps between surfaces of physical components. Specifically, dramatic increases in information bit density of recording media during recent years has necessitated correspondingly smaller head gaps to assure accurate transducing of information stored on the recording media. Heretofore, the flying height of a transducing head above a magnetic storage disk has been measured by substituting an optical head for the magnetic storage head and employing white light interferometry to obtain an indication of the spacing between the optical head and the disk surface. The interferometry method of proximity sensing involves analysis of a concentric ring pattern of interference bands known as "Newton's rings". Physical spacing between a spherical lens surface and another surface is related to the wavelength of light discernible by spectral color in the rings. In the context of physical gap measurement or proximity sensing, the minimum gap that can be discerned with interferometry alone is approximately 4.5 microinches.
Another characteristic of proximity sensing represented by the magnetic recording media art is that physical spacing of components within microinch tolerances is most commonly achieved by aerodynamic phenomenon in which relative movement of the components is required to maintain spacing. Thus, real time observation and measurement of the spacing gap is important to component calibration where all related operating parameters are to be accounted for. Prior attempts at adaptation of interferometry to real time measurement of the spacing between moving surfaces have been limited to computer processing of spectrophotometric data and, as such, have not attained a true real time indication of surface proximity.
It is also known that the proximity of a glass surface to the surface of another body will be revealed by frustration of total light reflection from the interior of the glass surface. If collimated light is directed through a glass body to the inside of a surface of the body at an angle greater than the critical angle, determined by the refractive index of the glass relative to the refractive index of the surrounding media such as air, the light will be totally reflected from the inside of the surface. The total reflection of the same light, however, will be reduced or frustrated by the close proximity to the outside of the same glass surface of another body or surface. Moreover, if the glass surface is a spherical surface of a large radius and the proximate surface of the outside body is planar, the frustration of total internal reflection will be revealed as a dark spot of radially decreasing density in an area of the internally reflected light. Although the phenomenon of frustrated total internal reflection has been used to indicate relative microspacing (U.S. Pat. Nos. 3,987,668, 4,286,468, and 4,322,979) or to modulate a light beam (U.S. Pat. Nos. 3,338,656, 4,165,155 and 4,451,123), it is not known that the phenomenon has been adapted to quantify microspacing increments nor to provide an image of microspacing topography throughout an area or field of interest.